The approach can be used in a number of forms. In Greenwood's (1973, 1975) work, for example, seven estimating equations for migration are used with seven jointly dependent variables. Five of the questions are structural and two are identities. The dependent variables include the number of labour force (CLF) (in Greenwood's original model, labour force relates specifically to the civilian labour force), out-migrants from the area (OM), the number of labour force inmigrants to the area (1M), income change in the area (11INC) (the symbol 11, capital 'delta', is conventionally used to represent a change in a variable over a specified period of time), employment change in the area (I1EMP), unemployment change in the area (I1U N EM P), labour force change (I1C LF), and natural increase of the labour force (N A TI NC). The seven equations are as follows: OM ./; (1M, I1INC, I1EMP, I1U NEMP, I1INCb, UNRb, CLFb,

I1INC I1EMP

e2) ./~(OM, 1M, I1EDU, I1GOvr, DEW, DNS, e3) .f4(OM, 1M, NATINC, INCb, I1EDU, I1GOvr, DEW, DNS,

where each of the variables is defined as follows. Dependent variables: OM is the outmigration over the period from the base date, b, to the final date b' ; 1M is the gross inmigration over the same period; 111 N C in the income change over the period; /).EM P is the employment change; /).UNEMP is the unemployment change; /).CLF is the labour force change, in total, and NATINC is the natural increase of the labour force.